1. Field of the Invention
The present invention generally relates to a magnetic recording medium that is built in a magnetic recording apparatus such as a hard disk drive. The present invention more particularly relates to a magnetic recording medium of which magnetic grains of a magnetic recording layer are prevented from expanding during epitaxial growth on an underlayer.
2. Description of the Related Art
A magnetic recording apparatus is an apparatus for recording a large amount of digital signals at low cost per bit. The magnetic recording apparatuses are widely used as secondary storage devices of personal computers.
The demand for the magnetic recording apparatuses for recording TV programs at home is expected to soar in conjunction with the start of digital television broadcasting in the near future. The capacity of the magnetic recording apparatus needs to be further increased for recording video signals of television. Cost per bit also needs to be reduced for consumer use.
The recording density of a magnetic recording medium needs to be increased because of the above reasons. It is an important object that, as the recording area for a bit is reduced, the size of magnetic grains needs to be reduced and made uniform, and the noise also needs to be reduced.
To achieve this object, a magnetic recording layer needs to be formed by cumulating fine magnetic grains the crystal axes of which are controlled, and a magnetic recording medium having such a magnetic recording layer needs to be fabricated.
However, a magnetic recording medium fabricated in a conventional general fabrication process is a structure in which polycrystalline layers are laminated by forming an underlayer and a magnetic recording layer in this order. The magnetic grains are cumulated on crystal grains of the underlayer without enough epitaxial growth in this fabrication process. A method is proposed in which magnetic grains are made fine by adding substance for adjusting grain diameter such as B and Ta.
However, if a magnetic layer is formed by sufficient (substantially complete) epitaxial growth in the deposition of magnetic grains on the underlayer, the aforementioned method in which magnetic grains are made small by adding substance such as B and Ta is unusable.
Accordingly, the underlayer needs to be finely grained first. The magnetic layer is formed on the fine-grained underlayer by the deposition of magnetic grains by sufficient epitaxial growth. This is, however, an indirect method.
The aforementioned indirect method causes a problem such that, since magnetic grains grow three dimensionally during the deposition on the underlayer, a crystal face with great lattice constant grows antecedently, and the magnetic grains become too large. Especially, this problem becomes apparent as the degree of vacuum of sputtering apparatuses is increased so as to promote epitaxial growth of layers.
Noise needs to be reduced in addition to preventing the magnetic grains from expanding.
Accordingly, it is an object of the present invention to provide a magnetic recording medium in which magnetic grains forming a magnetic layer are prevented from expanding in the deposition of the magnetic grains on an underlayer by epitaxial growth, and as a result, the recording density of the magnetic recording medium is increased. Another object of the present invention is to provide a magnetic recording medium of which noise is further reduced.